The problem with film cameras that add exposures to a partially exposed roll of film is a familiar one. After reinserting partially exposed film back into a camera, there is a need for a simple, reliable and inexpensive way of preventing a camera from creating double exposures, particularly when the film is reloaded after prior exposures have been made in a different camera.
If a 35 mm camera user wanted to remove a roll of film from a camera and reintroduce that roll of film into another camera, the camera user would have a great deal of difficulty in performing this task without producing any double exposures or wasting any film. The reason for the above is that exposures are usually started from different locations of the roll of film and oftentimes two different cameras will not wind or rewind the exact same length of film each time.
Normal 35 mm film has a continuous set of perforations along both edges of the film throughout its length. Exposures have no prealocated position on the roll of film. Thus, a mid-roll interrupt camera that uses 35 mm film has to not only find out which exposures have been made, but also where the exposures are located on the roll of film.
In 110 and 126 film, the perforations along the edge of the film uniquely identify each exposure area so that all camera's will place the same numbered exposure at the same location on the roll of film. The task remaining for the mid-roll interrupt camera is to mark the exposures that have been made and determine which exposures have not been made.
Most cameras require a camera user to place a roll of film in the camera and expose one or more frames of the roll of film. In a normal-wind camera, exposures are made on a roll of film as the film is pulled out of the cartridge. When all of the frames on the roll of film are exposed, the roll of film is wound back into the cartridge. The first exposure will be at the leader end of the roll of film and the last exposure will be near the cartridge spool. In a prewind camera the entire roll of film is pulled out of the cartridge (over to the take up side) before the first exposure is made. Then, the exposures are made as the roll of film is wound incrementally back into the cartridge. The first exposure will be near the cartridge spool and the last exposure will be at the leader end of the roll of film.
Mid-roll interrupt cameras have been developed to allow a camera user to remove a partially exposed roll of film and reintroduce that roll of film to the same or another camera, with exposure continuing on unexposed frames only. In a mid-roll interrupt camera no exposures will be lost and no double exposures will be allowed to take place. Typically, a mid-roll interrupt camera will utilize a roll of film that has magnetics on the film. The magnetics on the film is a magnetically sensitive layer on the roll of photographic film that allows equipment to write and/or read on that magnetically sensitive layer.
One type of mid-roll interrupt camera, (presence of signal type) writes information on a magnetically sensitive layer of a roll of film as it advances film after each exposure is made. Thus, a trail of magnetic data will be left near the location of each exposure. When a partially exposed roll of film is removed from one mid-roll interrupt camera and placed in another mid-roll interrupt camera, the other camera will read the magnetic data. The presence of written magnetic data will indicate that particular frames have been exposed and the absence of magnetic data will indicate that the frame has not been exposed.
Another type of mid-roll interrupt camera (decoded data type) writes information on a magnetically sensitive layer of the roll of film when the mid-roll interrupt feature is requested. When a partially exposed roll of film is removed from one mid-roll interrupt camera and placed in another mid-roll interrupt camera, the other camera will find that magnetic data and decode it to find the unexposed frames.